1. Field of the Invention
This invention relates to an optical glass for precision molding, capable of carrying out a precision molding at a low temperature and needing no polishing and grinding after the precision molding.
2. Description of the Prior Art
With lately marked progress in obtaining high performance and effective miniaturization of optical devices such as digital cameras, the requirements of miniturization and weight reduction for lenses composing such optical systems are increasing more and more and the use of aspheric lenses made by the precision molding method has become indispensable. Furthermore, a precisely molded, high refractive index and high dispersion glass has strongly been required so as to realize a very compacted optical system in various optical devices.
In the high refractive index, high dispersion optical glass of the prior art, a composition system containing a large amount of lead oxide is typical, which is stable because of having a high devitrification resistance and has been used for precision molding because of having a low transition point (Tg) and low yield temperature (At). For example, JP-A-1-308843, JP-A-7-247135 and JP-A-7-247136 respectively disclose an optical glass for precision molding containing a large amount of lead oxide.
However, the precision molding has generally been carried out in a reducing atmosphere so as to prevent a mold from oxidation, so if lead oxide is contained in the glass composition, the lead oxide on the glass surface is reduced and deposited as lead on the surface of a precision molding lens. The lead is evaporated by heating for the precision molding, a part of which adheres to the surface of the mold material to form a convex part which is then transferred to the surface of the precision molding lens as it is. When such steps are continuously repeated, the surface accuracy of the precision molding lens cannot be maintained. In addition, lead oxide has such a problem that it is harmful to the environment and human body. Accordingly, the glasses disclosed in the foregoing JP-A-1-308843, JP-A-7-247135 and JP-A-7-247136 are not suitable, nor practically used as an optical glass for precision molding.
On the other hand, in glasses described in JP-A-8-157231, JP-A-2003-300751 and JP-A-2003-335549, surely, lead oxide is not contained. In JP-A-8-157231, however, in place of the lead oxide, TiO2 is all incorporated, although defined as an optional component in claim 5, in order to obtain high refractive index, high dispersion properties as shown in Examples 7 to 11 (Cf. JP-A-8-157231, page 7, Table 2). Consequently, the resulting glass is very strongly colored. In JP-A-2003-335549, similarly, in Examples (Cf. JP-A-2003-335549, Table on pages 4 to 6) TiO2 is all incorporated. In an invention described in JP-A-2003-300751, TiO2 is an essential component. In the ordinary optical system, use of a single glass lens is not realistic and many optical systems are substantially composed of a number of lenses. Thus, it is desired that coloration of these glass lenses is rendered as little as possible. Accordingly, the glasses described in JP-A-8-157231, JP-A-2003-300751 and JP-A-2003-335549 are substantially caused to have high referactive index, high dispersion properties by TiO2, which cannot be said to be preferable from the standpoint of optical designing.
The glass described in JP-A-2001-058845 (U.S. Pat. No. 6,333,282B1) is a high refractive index, high dispersion optical glass of P2O5—Nb2O5—Bi2O3-alkali metal oxide type, the present inventors have invented, having a refractive index (nd) of at least 1.83 and Abbe number (νd) of at most 26.0. The refractive index (nd) of the optical glass herein described is only less than 1.89, and if the refractive index is at least this range, the devitrification resistance is deteriorated to render it unstable. This is the limit for the glass.